This invention relates to fusing toner images and more particularly to a heat and pressure roller fuser for fixing toner images to copy substrates.
The invention can be utilized in the art of xerography or in the printing arts. In the practice of conventional xerography, it is the general procedure to form electrostatic latent images on a xerographic surface by first uniformly charging a photoreceptor. The photoreceptor comprises a charge retentive surface. The charge is selectively dissipated in accordance with a pattern of activating radiation corresponding to original images. The selective dissipation of the charge leaves a latent charge pattern on the imaging surface corresponding to the areas not exposed by radiation.
After the electrostatic latent image is recorded on the photoconductive surface, it is developed by bringing a developer material including toner particles into contact therewith to thereby form toner images on the photoconductive surface. The images are generally transferred to a support surface such as plain paper to which they may be permanently affixed by heating or by the application of pressure or a combination of both.
One approach to thermal fusing of toner material images onto the supporting substrate has been to pass the substrate with the unfused toner images thereon between a pair of opposed roller members at least one of which is internally heated. During operation of a fusing system of this type, the support member to which the toner images are electrostatically adhered is moved through the nip formed between the rolls with the toner image contacting the heated fuser roller to thereby effect heating of the toner images within the nip. As will be appreciated, in a machine where duplex images are created both rolls may be heated. In either case, one of the rolls is usually referred to as the fuser roller while the other is commonly referred to as a pressure or back-up roll.
U.S. Pat. No. 4,570,044 discloses a basic induction heated roller fusing system as above. Unfortunately, in induction heated apparatus as such, the thermal time constant of the inductively heated fuser roll is hard to minimize, and mechanical support elements such as end caps, bearings, gears, and yokes that enclose the transformer core are undesirably heated extraneously.
In accordance with the present invention, there is provided a fusing apparatus for heating and permanently fusing toner powder images onto an image carrying sheet. The fusing apparatus includes a pressure roller; a closed loop magnetic flux carrying member positioned adjacent the pressure roller and including a first side and a second side opposite the first side. The first side is located between the pressure roller and the second side. The fusing apparatus also includes an electrically conductive wire wound about the second side forming a primary transformer coil having N1 number of turns. The primary transformer coil is connectable to an AC power supply source for inductively transferring AC electric energy to the first side. Importantly, the fusing apparatus includes a rotatable fuser roller forming a fusing nip with the pressure roller. The rotatable fuser roller has an electrically conductive layer and a rigid non-conductive core in the form of a ceramic tube underlying the conductive layer. The rotatable fuser roller is mounted around the first side of the closed loop magnetic flux carrying member and forms a secondary transformer coil inductively coupled to the primary transformer coil, and the conductive layer is inductively heated by power dissipated by current induced therein when the primary transformer coil is connected to the AC power supply source.